Diode lasers provide inexpensive, high-efficiency production of laser light in the near infrared spectrum. This laser light has a number of uses, including optical pumping of alkali metals for spin-exchange optical pumping. However, producing high power output beams with optimal properties from stacks of diode array bars that are well collimated and with uniform intensity has proven challenging.
Single-stripe diode lasers provide a diffraction-limited source with narrow spectral output. Because the source of the light is very small, particularly in one dimension, the light can be collimated with a micro lens, producing a narrow-spectrum, highly collimated beam. In order to achieve the very high powers required for some uses, including optical pumping, many diode stripes 10 are combined onto a single bar 11 as shown in FIG. 1. In order to achieve even higher power, as shown in FIG. 2, several bars 11 are utilized, as a stack, as well. The challenge is both (i) to control all of these stripes so that they lase at the same wavelength, and (ii) to direct the light from the stripes to illuminate a target, such as an optical pumping cell, with uniform intensity without gaps or shadows. In some cases, the optical pumping cell can be quite long, imposing the additional requirement that the light be well collimated.
A bar can have fifty to one hundred stripes, each emitting light out the end. The emission facet then has a surface of one micron by one hundred microns. The emitted light is diverging rapidly along a dimension transverse to the one micron thickness. This spread in emission angles is roughly ±45° at the diffraction limit. This axis is called the “fast axis.” The larger hundred micron dimension has an associated transverse divergence in the emission angles of ±10°, along the “slow axis.”
One existing method for directing several diode laser stripes toward a single target has been fiber-coupling. A small fiber is attached to each diode laser stripe on a bar (which may contain 20 to 30 stripes) and combined into a single output. These Fiber Array Packages (“FAP's”) can then be coupled to an external fiber to deliver the light to the optical pumping cell. When the light emerges from this external fiber, it can be combined with light from other FAPs. It is then passed through a linear polarizing cube, converted from linear to circular using quarter wave plates, and then directed onto the experiment.
The spectral output of these FAPs depends on the distribution of wavelengths of the individual diode laser stripes. Typically, the process can be controlled such that the r.m.s. difference among the wavelengths of light emitted by the stripes tuned to 795 nm is less than two nanometers. This width is not optimal for certain applications, including optical pumping of alkali metals, as the width of the rubidium absorption spectrum is less than one nanometer (in some cases less than one-tenth nanometer (one angstrom)).
Moreover, reference is made to the definition of etendue, the geometric capability of an optical system to transmit radiation. The numeric value of the etendue is a constant of the system (apart from aberrations) and is calculated as the product of the opening size and the solid angle from which the system accepts light. In most practical situations it is approximately the integral over a surface area transmitting the light multiplied by the solid angle into which the light is being transmitted. The etendue of a single diode strip is diffraction limited along the fast axis, but not diffraction limited along the slow axis. The etendue of fiber coupled high power systems is far from diffraction limited. Consequently, the light intensity that can be delivered to a long optical pumping cell is reduced with fiber coupled systems.